1. Field of the Invention
The present invention relates to a water heating system, and more particularly to an improvement of coefficients of performance by an active control of the refrigeration cycle of the system.
2. Description of the Prior Art
Recently, a keen attention has been paid to a heat pump water heating system, since it is considered as a system which is not inferior in its energy saving capability to the most popular natural gas water heating system, because of its unique heat transporation function. A heretofore known heat pump water heating system as shown in FIG. 1 has the basic functions of absorbing heat from ambient air, heating up refrigerant by means of a heat pump and storing the heated water in a water tank.
Referring now to FIG. 1, the system function is described specifically. An evaporator 1 absorbs heat from the ambient air blown in by a fan 2 to evaporate liquefied refrigerant. A compressor 3 compresses the evaporated refrigerant to raise the pressure and temperature thereof. A condenser 4 liquefies the refrigerant by heat exchange. An expansion valve 5 causes an adiabatic expansion of the refrigerant to decrease the pressure and temperature thereof. A heat exchanger 6 receives heat from the condenser 4 to heat water, and a circulating pump 7 feeds heated water into a hot water tank 8. However, in this type of prior art heat pump system, in which the difference of the temperature between ambient air and water in the tank 8 is not constant and both the rotational speed of compressor 3 and the opening of the expansion valve 5 are almost fixed, it has been difficult to maximize the coefficient of performance over the operational range of variation of the temperature difference.
As another type of energy saving system, a solar hot water heating system as shown in FIG. 2 is gaining popularity. Referring now to FIG. 2, a solar collector 11 heats water as refrigerant, and the hot water is fed into a heat exchanger 14 by means of a circulating pump 13. Water in a water heating tank 16 is heated by the heat exchanger 14. The hot water from the tank 16 is fed through a pump 17 into a water storage tank 18, where an auxiliary heat source 19 heats the hot water at a suitable temperature. Since this system uses the solar radiation as its heat source, its energy saving capability is excellent but the volume of hot water obtained thereby is limited. Further, solar heat cannot be obtained in cloudy or rainy days, and the heat dissipation from the solar collector 11 to ambient air is increased during winter season. Consequently, the system cannot be used without auxiliary heat sources such as natural gas, fuel oil, electric resistance heater, etc., so that the initial investment of the system is increased, which offsets the advantages of energy saving capability obtained by the solar radiation. In addition, in both systems shown in FIGS. 1 and 2, the energy losses cannot be neglected associated with the circulating pumps, additional heat exchangers, and pipes etc.